My reaction on first hearing about this paper.
Having gotten that out of the way, I sat down and read the paper – which was published in PNAS. The paper was written by Yoel Rak, Avishag Ginzburg, and Eli Geffen. The short version is that Australopithecus afarensis displays mandibular traits that indicate it is too derived to be a human ancestor. The longer version is a little bit more interesting, although I am inclined to be somewhat skeptical.
Let’s start at the beginning. Below is a picture of a human mandible:
The anatomy we are interested in is in the upper right hand corner of the bottom picture. Specifically, the coronoid process, the mandibular notch and the condyle. In humans, the condyle is taller than the coronoid process and the deepest part of the mandibular notch is situated anteriorly (towards the front of the jaw). According to Aiello and Dean’s An Introduction to Human Evolutionary Anatomy the reverse is true in apes. Rak et al take a slightly different view. The analyzed the shape of the mandible in 146 extant primates, including gorillas, chimps (common and pygmy), orangutans and humans. According to their analysis, the above species fell into two groups based on their mandibular anatomy. The first group includes humans, chimps and orangs. The second consisted solely of the gorilla. Then they threw some fossil specimens into the mix. The fossil specimens include the recently (2002) discovered A. L. 822-1, MAK-VP 1/83 (both Au. afarensis), SK 23, Sk 34 (both Au. robustus) and GWM5sw/P56 (Ardipithecus ramidus). Some of which are pictured below (along with a key):
(Ramal morphology in Au. afarensis and extant primates. (Top) Left mandibular ramus and right mandibular ramus (horizontally flipped) of Au. afarensis specimen A. L. 822-1 and left mandibular ramus of a gorilla. (Middle) Left mandibular ramus of Au. afarensis MAK-VP 1/83 specimen; fragment of left mandibular ramus of Au. afarensis specimen A. L. 333-100; and mandibular ramus of Au. robustus specimen SK 23. (Bottom) Mandibular ramus of a chimpanzee, an orangutan, and H. sapiens. (Scale bar: 5 cm.) Note that the upper end of the ramus in all of the specimens above the white line resembles that of a gorilla (particularly in the shape of the coronoid, the great percentage that the coronoid base constitutes of the ramal width, the confined appearance of the mandibular notch, and the small percentage that the notch area constitutes of the ramal area). The limited reconstruction of the coronoid process on the left ramus of A. L. 822-1 is based on the corresponding preserved area on the right ramus and vice versa.)
Rak et al argue that the condition seen in chimps, humans and orangs represents the primitive condition and that seen in gorillas and Au. afarensis are therefore derived. Furthermore, they argue that the condition seen in Au. afarensis is too similar to that seen in Au. robustus for Au. afarensis to be a direct human ancestor. To support that contention they also point out that the condition in Ardipithecus ramidus is similar to chimps, humans and orangs. Interestingly enough, the Dikika find displayed some gorilla-like traits.
Having said all that, I am somewhat skeptical. One reason is that Au. afarensis has always been considered ancestral to both humans and robust australopithecines, so the fact that Rak et al found another trait that connects them doesn’t surprise me. Nor does it, in my opinion, rule them out as human ancestors. What it does do is reinforce the highly transitional nature of Au. afarensis. Another reason is that starting with the announcement of the discovery of Au. afarensis there have been a number of suggestions that the morphology is so varied that two species must be represented. This paper even argues that the assemblage represents two species – a hominid and a pongid (similar to Dryopithecus). The fact that Au. afarensis has some traits similar to the gorilla doesn’t really surprise me (I would have been surprised if they had orang or gibbon traits). I’ll repeat what I said in a previous post:
This issue of taxonomic blurring is only going to loom larger in paleoanthropology as more fossils are discovered. Think of the reptile/mammal transition where deciding whether a given specimen is more properly considered a reptile or a mammal can be difficult (technically this is a somewhat misleading way of saying it. Mammals and mammal like reptiles are members of the Synapsida and technically mammal like reptiles are called nonmammalian synapsids and the nonmamalian synapsids share some primitive features with the common ancestors of reptiles and synapsids. Essentially, the reptiles and synapsids form a sister group relationship rather than an ancestor/descendant relationship.) The point here is that as the hominid family tree proliferates we are going to see a wide variety of fossils with a wide variety of traits that are going to blur the boundaries not only between Australopithecus and Homo but within those taxonomic categories as well.
Or, in this case, between members of the Hominidae, but then I could be in denial…